This invention relates to magnetic recording media, and more particularly, to magnetic recording media comprising a substrate, a non-magnetic undercoat on one surface of the substrate, a magnetic layer on the undercoat, and a backcoat on the other surface of the substrate.
The most common and simple form of magnetic recording medium is magnetic recording tape comprising a substrate or film and a magnetic layer formed thereon. It was a common practice in the prior art to prime a substrate such as a polyester film with a resin solution to form an undercoat on the substrate to improve the bond between the substrate and the magnetic layer formed thereon.
After a resin solution is applied to form the undercoat, a magnetic lacquer of magnetic particles and a binder in an organic solvent is applied thereon to form the magnetic layer. The undercoat is affected by the organic solvent of the magnetic lacquer such that the undercoat is swelled to penetrate through the overlying magnetic layer until exposed at the surface. If the resin of the undercoat is thermosetting, unreacted portions remaining after a heat treatment will cause the turns or layers to be undesirably adhered when the magnetic recording medium is wound into a roll or placed one on top of the other, eventually deteriorating the surface properties of the medium.
To obviate this problem, it was proposed to apply a resin containing a radiation-sensitive unsaturated double bond onto a substrate and expose the resin to radiation for crosslinking and polymerization before a magnetic layer is formed (see Japanese Patent Application Kokai No. 57-40747).
Electric charges accumulating during the application of an undercoating solution result in a non-uniform undercoat, for example, having streaks. It is thus necessary to allow such electric charges to escape away. Electric charges also prevent the film to be coated from smoothly travelling, also resulting in a non-uniform undercoat and disturbing the winding of the film into a compact roll. It is thus necessary to control such electric charges.
At present, magnetic tape is used in a variety of applications including audio, video, and computer applications. The quantity of information to be recorded in the medium is progressively increased and thus, the requirement of increased recording density is imposed on magnetic recording media.
In the existing recording mode using magnetic heads, the spacing loss between the tape and the head is represented by the formula: 54.6 d/.lambda. dB where d is a tape-to-head distance and .lambda. is a recording wavelength. This formula indicates that short wavelength recording promising a higher recording density undergoes a larger loss of output by spacing than long wavelength recording. Then, even small foreign particles on the tape surface are detectable as dropouts.
The factors believed to bring out dropouts include release of magnetic particles form the magnetic coating surface of the magnetic tape due to deterioration of the magnetic coating caused by the repeated application of stresses, electrostatic adhesion of pieces worn from the substrate and dust to the substrate surface, and transfer of such worn pieces and dust to the magnetic layer surface. A variety of methods have been proposed in order to eliminate these unfavorable factors. For example, it is proposed to apply a lacquer of carbon black or graphite in an organic binder onto that surface of the substrate remote from the magnetic layer, that is, the back surface to form a backcoat to render the substrate more tough for minimizing abrasion or wear of the substrate. Such treatments including backcoating are effective in reducing the tendency of dropout increasing with repeated travels, but not to a satisfactory extent. There is the need for further reducing dropout. How dropouts are induced in magnetic tape is described in Japanese Patent Application No. 56-54362 of the same assignee as the present application. The binders used are usually thermosetting resins. In an ordinary coating process, tape is coated with a backcoat and then wound into a roll before a heat treatment is effected for curing. This means that the backcoat is fragile at the end of coating process because the curing reaction is not initiated. Since the backcoat is in firm contact with the magnetic layer in the roll form, particles of the inorganic filler such as carbon black and graphite present on the surface of the backcoat tend to transfer to the opposed surface of the magnetic layer in contact therewith. The thus transferred particles cause dropouts and head gap jamming. It is believed that this phenomenon also applies to thermoplastic resins. The number of dropouts is not satisfactorily small at the initial stage or after several travels for the reason mentioned above although the provision of a backcoat is effective in retarding dropout from increasing with repeated travels.
In order to eliminate the above-mentioned inconvenience in the backcoat forming process, it is possible to form a backcoat by applying a lacquer composition of an inorganic filler such as carbon black and graphite and a binder in the form of a radiation-curable resin (resin capable of crosslinking and curing upon exposure to radiation) onto a substrate to form a backcoat, applying radiation from an active energy ray source to effect a curing treatment or carrying out a surface treatment on the as-coated substrate, and then a curing treatment, thereby inducing three dimensional crosslinking in the backcoat to achieve a tough backcoat, and thereafter winding the tape into a roll. This process reduces the dropout due to the transfer of filler particles as mentioned above. In this process, the crosslinking reaction proceeds to the end in the backcoat before the tape is wound into a roll. Even when the backcoat is brought in close contact with the magnetic layer by winding, the transfer of filler particles from the backcoat to the magnetic layer is prohibited.
A satisfactory anti-static effect is not obtained unless inorganic fillers such as carbon black and graphite are blended in resins in larger amounts. However, blending large amounts of inorganic fillers gives rise to a problem in the surface roughness of the backcoat, which in turn, results in a reduction in the output of the magnetic layer.